hippocampus

hippocampus is a major component of the brain of humans and other vertebrates. It belongs to the limbic system and plays essential roles in the consolidation of information from short-term memory to long-term memory, and in spatial memory that enables navigation. Structurally, it is a paired structure, with one located in each hemisphere of the brain, and is shaped like a seahorse, from which its name is derived.

Anatomy and structure

The hippocampus is located within the medial temporal lobe, beneath the cortical surface. It is anatomically connected to structures such as the entorhinal cortex, the fornix, and the amygdala. The classic archicortical structure is divided into several subfields, including the cornu ammonis areas (CA1, CA2, CA3), and the dentate gyrus. These regions form a largely unidirectional trisynaptic circuit that is critical for its function. Input primarily arrives via the perforant path from the entorhinal cortex to the dentate gyrus. Major output pathways project through the fornix to targets like the mammillary bodies and the anterior nucleus of the thalamus.

Function and mechanisms

The primary functions involve memory formation, spatial navigation, and behavioral inhibition. A key mechanism underlying these functions is synaptic plasticity, most famously exemplified by long-term potentiation, a process first thoroughly investigated in the hippocampus by Timothy Bliss and Terje Lømo. This plasticity is believed to be the cellular basis for learning and memory. The hippocampus also exhibits distinct patterns of neural activity, such as theta oscillations and the firing of place cells, which were discovered by John O'Keefe and are integral to forming a cognitive map of the environment.

Role in memory and learning

The hippocampus is crucial for forming declarative memories, which include both episodic memory (personal experiences) and semantic memory (general knowledge). Its role is often described as a temporary storage site before memories are consolidated to the neocortex. Seminal evidence for this role came from studies of patient Henry Molaison (known as H.M.), who, following bilateral medial temporal lobe resection, developed severe anterograde amnesia. Research by Brenda Milner on this case fundamentally advanced the understanding of memory systems. The structure is also vital for spatial learning, as demonstrated in tasks like the Morris water maze.

Clinical significance

Damage or dysfunction of the hippocampus is implicated in several neurological and psychiatric disorders. It is one of the first brain regions affected in Alzheimer's disease, leading to characteristic memory deficits. Atrophy is also observed in temporal lobe epilepsy, schizophrenia, and major depressive disorder. Stress and elevated levels of glucocorticoids (e.g., cortisol) can damage hippocampal neurons, potentially linking chronic stress to memory impairments. Furthermore, the hippocampus can be damaged by cerebral hypoxia, encephalitis, or limbic encephalitis.

Evolution and comparative anatomy

The hippocampus is an evolutionarily ancient structure, with homologues present in all vertebrates. The pallium of birds and reptiles contains a region considered homologous, often involved in spatial memory. In certain mammals, such as london taxi drivers studied by Eleanor Maguire, structural changes in the hippocampus correlate with navigational expertise. The size and complexity of the hippocampus can vary significantly across species; for example, it is exceptionally large and specialized in food-storing birds like clark's nutcracker and in mammals with high spatial demands, such as brown bats.

Category:Neuroanatomy Category:Limbic system Category:Memory